Create direct TCP proxy

A command that spins up a TCP proxy capable of connecting using an ntor/obfs4 bridge should be created. This will be similar to https://gitlab.com/yawning/obfs4/-/tree/master/obfs4proxy but we can keep things a bit simpler since we only care about supporting one proxy type (to start) and one pluggable transport.

We'll need to decide if this is a separate module to avoid downloading dependencies; classic Go packaging suggests that mixing main packages with libraries is okay, but Go Modules makes this a bad idea. Should we merge our obfsvpn stuff back into the shapeshifter repo, create a separate repo for both, have two different modules in the same repo?